Chapter 1.1: Discoveries That Changed Our Perspective

The early 20th century brought groundbreaking discoveries that transformed our understanding of the cosmos. Edwin Hubble was the first to identify other galaxies and subsequently demonstrated that not only were these galaxies situated millions of light-years away, but they were also receding from one another. The farther apart they are, the faster their separation!

Numerous explanations have been proposed for this cosmic expansion, but only one aligns closely with both empirical observations and theoretical calculations: the universe is expanding! As astronomers continued to discover new galaxies, the evidence supporting this expansion grew stronger.

But what drives this expansion? Several hypotheses have been suggested:

• Is it due to a rotational force around a central point, causing centrifugal expansion?
• Could spatial oscillations be propagating through the universe, leading to expansion in observed regions while contracting in others?
• Might the gradual creation of new matter be a factor?
• Or is it the case that the universe once existed in an incredibly dense, hot state that subsequently expanded?

The latter scenario aligns with the Big Bang theory.

Chapter 1.2: Predictions of the Big Bang Theory

If the Big Bang theory holds true, it allows us to make several predictions about the universe's structure. For instance, if the universe originated from an infinitely hot and dense state and expanded, then observing distant regions of space should reveal younger galaxies characterized by blue stars.

1. Additionally, there must have been a moment when the universe cooled sufficiently for the first atoms to form from ionized plasma. We should still be able to detect the radiation emitted by the universe's matter during this period.

This radiation, known as relic radiation, was initially predicted by George Gamow and later discovered by Arno Penzias and Bob Wilson.

2. Finally, if the universe began from a highly dense, hot state devoid of atoms, we can calculate the elemental composition that should exist throughout the universe. Heavy elements form through thermonuclear reactions within stars, meaning the early universe consisted primarily of light elements like hydrogen and helium.

These proportions have been confirmed in distant galaxies, which reflect the universe's early stages, showing roughly 75% hydrogen and 25% helium, with trace amounts of deuterium, helium-3, and lithium.

For more than half a century, no alternative theory has managed to explain these phenomena with the same level of precision or alignment with observational results as the Big Bang theory has. Thus, there are currently no credible alternatives to it.

In the video "What if there was no Big Bang? Alternatives to the Big Bang Theory," the host explores various cosmological theories that challenge the Big Bang, discussing their shortcomings and contrasting them with the prevailing theory.

In "Five (Failed) Alternatives to the Big Bang - Ask a Spaceman!" the presenter delves into five notable theories that attempted to explain the universe's origin but ultimately fell short, reinforcing the Big Bang's dominance.

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